Optionally allow an output property.
Optionally allow an output property.

Unblocks #269.

Really we should use the output schema to validate the keys
are one of the known output types.

But I can't figure out how to ref https://github.com/json-schema-org/json-schema-spec/blob/master/output/schema.json
plus that URL is going to change.

So... later.

json-schema/JSON-Schema-Test-Suite,json-schema-org/JSON-Schema-Test-Suite